Scientists struggling to deal with damaged reactors in Japan

Japanese nuclear experts were struggling to control three damaged reactors amid fears they could go into meltdown tonight.

Cooling water levels dropped suddenly in one, twice leaving the uranium fuel rods completely exposed and increasing the danger.

Water levels were restored after the first decrease but the rods remained exposed after the second episode.

Earlier an explosion tore through the building housing a different reactor.

The cascading troubles in the Fukushima Dai-ichi plant compounded the immense challenges faced by the government in the wake of the tsunami disaster.

The biggest problem facing engineers was the drop in water levels at the plant’s Unit 2.

“Units 1 and 3 are at least somewhat stabilised for the time being,” said a Nuclear and Industrial Agency spokesman “Unit 2 now requires all our effort and attention.”

A government spokesman said there were signs that the fuel rods were melting in all three reactors, all of which had lost their cooling systems.

“Although we cannot directly check it, it’s highly likely happening,” he said.

Some experts would call that a partial meltdown. But others reserve the term for when nuclear fuel melts through a reactor’s innermost chamber but not through the outer containment shell.

By contrast, a complete reactor meltdown, where the uranium core melts through the containment shell, would release a wave of radiation and result in major, widespread health problems.

Earlier the building surrounded Unit 3 exploded in a similar hydrogen blast to the one that destroyed the housing around Unit 1 on Saturday.

The blast actually lessened pressure building inside the reactor, and officials said the all-important containment shell – thick concrete armour around the reactor – had not been damaged.

Radiation levels remained within legal limits, although anyone left within 12 miles of the scene was ordered to remain indoors.

The explosion injured 11 workers and came as authorities were trying to use sea water to cool the three reactors.

While four Japanese nuclear complexes were damaged in the wake of Friday’s disaster, the Dai-ichi complex, which sits just off the Pacific coast and was badly hammered by the tsunami, has been the focus of most of the worries.

Operators knew the sea water flooding would cause a pressure buildup in the reactor containment vessels – and potentially lead to an explosion – but felt they had no choice if they wanted to avoid complete meltdowns. Eventually, hydrogen in the released steam mixed with oxygen in the atmosphere and set off the two blasts.

Japan’s meteorological agency did report one good sign. It said the prevailing wind in the area of the plant was heading east into the Pacific, which experts said would help carry away any radiation.

Across the region, though, many residents expressed fear over the situation.

People in the port town of Soma had rushed to higher ground after a tsunami warning Monday – a warning that turned out to be false alarm – and then felt the earth shake from the explosion at the Fukushima reactor 25 miles away. Authorities there ordered everyone to go indoors to guard against possible radiation contamination.

“It’s like a horror movie,” said 49-year-old Kyoko Nambu as she stood on a hillside overlooking her ruined hometown. “Our house is gone and now they are telling us to stay indoors.

“We can see the damage to our houses, but radiation? ... We have no idea what is happening. I am so scared.”

International scientists say there are serious dangers but little risk of a catastrophe like the 1986 blast in Chernobyl, where the reactor did not have a containment shell.

Some analysts said the length of time since the nuclear crisis began indicated that the chemical reactions inside the reactor were not moving quickly toward a complete meltdown.

“We’re now into the fourth day. Whatever is happening in that core is taking a long time to unfold,” said Mark Hibbs, of the Carnegie Endowment for International Peace. “They’ve succeeded in prolonging the timeline of the accident sequence.”

But he said the Japanese appeared unable to work out what was going on deep inside the reactor. In part, probably because of the damage done by the tsunami.

“The real question mark is what’s going on inside the core,” he said.